Kanter for Williams is an interesting proposition. I would probably stick with Kanter because I'm not convinced Williams is a great option at SF. Also he seems redundant to Sap.
I think Williams is better now, as far as future goes who knows. But I agree with your point Williams is not a 3 yet and maybe never will be. He is redundant of Milsap but if you could have 2 Milsaps on the court I think I might take it.